This document discusses several patterns of non-Mendelian inheritance including incomplete dominance, co-dominance, polygenic traits, pleiotropy, epistasis, and environmental effects on phenotypes. Mendel's laws are generally correct but real life genetics is more complex. Non-Mendelian inheritance can occur when phenotypes do not match genotypes, multiple alleles interact, single genes affect multiple traits, or environmental factors influence gene expression.

1. Patterns of inheritance:
Non- Mendelian inheritance
Prepared by
Samira Fattah
Assis. Lec.
College of health sciences-HMU
Lecture 3

2. Mendel’s Laws Not Perfect
•Shortly people began to notice that not all traits are
“Mendelian”
–This means, they do NOT follow Mendel’s laws

3. Mendel’s Laws Not Perfect
–Was he just plain wrong?
• Truth is, his laws are correct and did explain how
genetics works
– Real life is just more complicated than peas!

4. Altering Mendel’s Ratios
Two different types of complications:
1. Genotypic ratios follow Mendel’s laws, but phenotypes do
not
• Somehow the underlying genotypic ratios are hidden
2. Mendel’s laws do not apply
• Both genotypes and phenotypes are not following
Mendel’s laws

5. Incomplete dominance
• Neither allele is dominant and heterozygous
individuals have an intermediate phenotype.
• For example, A cross between red and white
flowered plants produced plants with intermediate
flower colour i.e. pink colour in F1 and a modified
ratio of 1 red: 2 pink: 1 White in F2.

6. F1 generation
F2 generation
CRCR
All CRCW
CWCW
1 : 2 : 1
Gametes
Gametes
CR
CW
CR CW
CRCR CRCW
CRCW CWCW
Incomplete Dominance

9. Co-dominance
Example : human blood type
3 different alleles (= versions of genes coding for one trait)
include:
- IA
- IB
-and i
that code for A, B, and “O” blood types.

10. Genotype Phenotype
• IAi or IAIA  Type A blood
• IBi or IBIB  Type B blood
• ii  Type O Blood
• IAIB  Type AB blood (Co-dominant)

12. Polygenic Traits
• traits are not controlled by a single gene locus, but by the
combined interaction of many gene loci.
• characters show a range of continuous phenotypes instead of
discrete, defined phenotypes
• In this case many genes have an additive effect. The
characteristic or trait is the result of the combined effect of
several genes.

13. Ex: human skin color, eye color, hair color and
height.
Pigmentation in humans is controlled by at least
three (3) separately inherited genes.

15. Eye color is influenced by many genes coding
for different kinds of pigment as well as where
in the iris those pigments are found .

17. Pleiotropy
• Alleles at a single locus may have effects on two or
more traits.
• When a single gene affects the expression of more than
one phenotypic character, the phenomenon is known
as pleiotropy (from the Greek pleio meaning "more"
and "trop, meaning "change").

18. Examples of pleiotropsm
• Sickle cell disease
• Homozygotes have two copies of abnormal gene leading
to formation of crescent shaped or sickle shaped
hemoglobin (Hbs).
• These cells have very low oxygen carrying capacity. Thus
impair circulation. This often led to renal failure, cardiac
failure and thrombosis.

20. • Phenylketonuria
• One of the most widely examples in pleiotropy in humans
• Inborn metabolic disorder in which the homozygous recessive
individual lacks the enzyme Phenylalaine hydroxylase needed
to change phenylalanine (amino acid) to tyrosine (amino
acid).
• Lack of the enzyme phenylalanine hydroxylase is due to the
abnormal gene on chromosome 12.

21. • In children, symptoms are mental retardation,
decrease pigmentation of hair and skin and eczema.
• The heterozygous individuals are normal but carriers.

22. Epistasis
• One gene effecting or masking another gene
• or
• •Two genes controlling same phenotype
• •Mendel’s Laws are still working for each individual gene,
but phenotype is not determined by that single gene’s
genotype alone

23. Red hair
• Let's look at the example of red hair color in human beings.
• Blond, brown, and black hair in humans is determined by the
amount of a pigment called eumelanin.
• Without much eumelanin, you get blond hair; an intermediate
amount will be brown; a lot will get you black hair.

24. • A second pigment called pheomelanin makes red hair.
But usually people don't build up a lot of that because
of a totally different gene called MCR1.
• MCR1 allows the conversion of pheomelanin into
eumelanin, making redheadedness rare. The genes for
eumelanin and MCR1 interact to produce one single
phenotype.
• So why do we ever get redheads?

25. • Well, a certain variation of the MCR1 gene will stop the
conversion of pheomelanin into eumelanin, allowing the
build up of pheomelanin in the hair, which leads to red hair.

26. • Albinism
• Albinism, a lack of pigment resulting in pale white
individuals, is another example of epistasis.
• The albino condition occurs due to an entirely different
gene than the genes that encode skin color and tone.
• If the albinism gene is present, the organism will not have
any pigment - no matter what skin color is encoded by
other genes.

27. • an important protein named tyrosinase is necessary for the
production of the pigment melanin.
• A gene named TYR codes for tyrosinase.
• A variation of the TYR gene has a mutation that code for a
non-functional tyrosinase that dosent work.
• If a person only has nonfunctional tyrosinase, then no melanin
will be made in the body. Even though
that person’s genetic code might have
genes for dark skin, if there is no
pigment to make it with, the person
will be an albino.

28. Effects of Environment on Phenotype
• Genes play an important role in determining an
organism’s characteristics.
• However, for many characteristics, the
individual’s phenotype is influenced by other
factors as well.
• Environmental factors, such as sunlight and food
availability, can affect how genes are expressed in
the phenotype of individuals.

29. Environmental Effects on Plant Phenotype
• Hydrangea macrophylla
• Action of gene
responsible for floral
color is influenced by
soil acidity
• Flower color ranges
from pink to blue
Acid soil Alkaline soil

30. in human:
• Genes are a major determinant of human skin color.
• However, exposure to ultraviolet radiation can
increase the amount of pigment in the skin and make
it appear darker.